1. Field of the Invention
The present invention relates to a cordless power tool using a battery pack as a power source.
2. Description of the Related Art
Battery-powered tools, such as impact drivers, drills, drivers, or circular saws, have been extensively used because they are easy to use and no power source cables are required. To increase the usability of such tools, various functions are required to be added. A discharge voltage limiter (hereinafter referred to as “DVL”) function is one of the functions required for the battery-powered tools. The DVL function is for controlling the battery pack so as to halt discharging when the battery voltage has dropped to a predetermined voltage.
The battery pack with little charges remaining as a result of continuous use can be reused if the use of the battery pack is halted for a certain period of time to wait for recovery of the battery voltage. Because the recovered battery pack has a power to tighten further several screws, the tool is frequently used in such a manner. However, if this is done, the battery pack is discharged to a deeper level. Cells making up the battery pack are liable to be damaged if they are fully discharged. To prolong the lifetime (repetitive use number) of the battery pack, it is desirable to recharge the battery pack before it reaches a full discharge condition.
One of the cells of the battery pack is liable to be short-circuited or malfunctioned, or the internal resistance of the cell is abnormally increased when the battery pack is repetitively used a number of times. The use of such battery pack remarkably lowers the rotational speed of a motor of the tool. Operation of the tool, particularly an impact driver in which tightening torque is proportional to the rotational speed of the motor, cannot be performed properly with such a defective battery pack.
The DVL function is required in order to dissolve the above-described problems. Japanese Patent Application Publication No. 5-123975 proposes a DVL function but is involved with the following problems.
Driving the tool lowers the battery voltage due to various resistances distributed over the current flowing path. There exist resistances in a main switch, contact portions between the battery pack and tool, each cell of the battery pack having an internal resistance, and connection plates interposed between the cells. However, in the DVL function according to the conventional art, the fact that the battery voltage is greatly lowered at the time of start-up of the motor is not taken into consideration. Further, the DVL is activated with respect to the battery packs having a residual amount of charges.
A load current differs depending upon whether a load imposed on the battery pack is heavy or not, so that there exist a difference in the level of the battery voltage lowered due to the circuit resistance. Nevertheless, because the limit voltage is fixed, the battery pack is deeply discharged in the case of light load whereas the battery pack is not so discharged in the case of heavy load. The fact that the limit voltage is fixed does not allow the battery pack to intentionally deeply discharge for the purpose of activating the battery pack.
Even if the DVL is once actuated, the battery voltage can recover if the battery pack is left unused for a certain period of time. Using the recovered voltage, it is possible to rotate the motor for a short period of time from the closure of the main switch to the actuation of the DVL. Small-size screws can be tightened using the residual power of the battery pack, however, the battery pack results in a deeply discharged condition.
With the conventional DVL technology in which the residual capacity of the battery pack is detected based on the battery voltage, a problem exists in that detection of the residual capacity of the battery pack cannot be performed with high accuracy.